Aromatic polyester polyols are commonly used intermediates for the manufacture of polyurethane products, including flexible and rigid foams, polyisocyanurate foams, coatings, sealants, adhesives, and elastomers. The aromatic content of these polyols contributes to strength, stiffness, and thermal stability of the urethane product.
Commonly, the aromatic polyester polyol is made by condensing aromatic diacids, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Bio-renewable content alone can be misleading as an indicator of “green” chemistry. For example, when a food source such as corn is needed to provide the bio-renewable content, there are clear trade-offs between feeding people and providing them with performance-based chemical products. Additionally, the chemical or biochemical transformations needed to convert sugars or other bio-friendly feeds to useful chemical intermediates such as polyols can consume more natural resources and energy and can release more greenhouse gases and pollutants into the environment than their petro-based alternatives in the effort to achieve “green” status.
Waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers), provide an abundant source of raw material for making new polymers. Usually, when PET is recycled, it is used to make new PET beverage bottles, PET fiber, or it is chemically transformed to produce polybutylene terephthalate (PBT). Other recycled raw materials are also available. For example, recycled propylene glycol is available from aircraft or RV deicing and other operations, and recycled ethylene glycol is available from spent vehicle coolants.
Urethane formulators demand polyols that meet required specifications for color, clarity, hydroxyl number, functionality, acid number, viscosity, and other properties. These specifications will vary and depend on the type of urethane application. For instance, rigid foams generally require polyols with higher hydroxyl numbers than the polyols used to make flexible foams.
Polyols suitable for use in making high-quality polyurethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc, titanium, or tin. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat. No. 5,502,247). Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. 36 (1997) 1373).
Most frequently, ethylene glycol is used as the glycol reactant for glycolysis. This is sensible because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature.
Lignin is a principal component of vascular plants, contributing 20-30 wt. % to dry softwoods and hardwoods. Lignin is an amorphous, polyphenolic material produced from an enzyme-catalyzed polymerization of coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol. Industrial lignin is a by-product of the pulp and paper industry. Two types, kraft lignin (also called “alkali lignin”) and lignosulfonates, are available commercially. Organosolv lignins, a more soluble form of lignin, can be obtained by pulping wood with solvents such as alcohols or aqueous acetic acid, but organosolv lignins have not yet become widely available. Lignosulfonates have been used as water reducers for concrete and as dispersants for gypsum wallboard. Alkali lignins generally have lower molecular weight, narrower molecular weight distribution, and lower water solubility (except at higher pH) compared with lignosulfonates. Once sulfonated, however, alkali lignin is generally useful in a variety of dispersant applications. So far, organosolv lignins have had only limited practical utility.
Lignins have been incorporated into polyurethane foams and other products, typically by including the lignin as a filler or additive (see, e.g., Ton-That et al., “Biopolyols Containing Lignin for PU Applications,” CPI Polyurethanes Technical Conference, Sep. 22-24, 2014, Dallas, Tex., and U.S. Pat. Nos. 3,519,581 and 4,987,213). Lignins have also been modified by reaction with maleic anhydride, propylene oxide, or other reactants to give polyetherester intermediates for polyurethanes (see, e.g., U.S. Pat. No. 4,017,474).
Tannins occur naturally in plant species and are another type of polyphenolic biomolecule. Most tannins derive from gallic acid, flavone, or phloroglucinol. Tannins have been suggested as heat stabilizers for thermoplastic polyesters (see U.S. Pat. No. 6,395,808).
Lignins and tannins have not been reacted with glycolized thermoplastic polymer intermediates to produce polyols useful for polyurethanes.
Improved polyols are needed. In particular, the urethane industry needs sustainable polyols based in substantial part on recycled polymers such as the practically unlimited supply of recycled polyethylene terephthalate. Polyols with high recycle content that satisfy the demanding color, clarity, viscosity, functionality, and hydroxyl content requirements of polyurethane formulators would be valuable.